The present invention relates to a reflective color display device employing a light scatter liquid crystal display element and a method for manufacturing same, and more particularly to a reflective liquid crystal display device utilizing a polymer network liquid crystal containing a liquid crystal and polymer resin phase-separated through ultraviolet ray irradiation and a method for manufacturing same.
Liquid crystal display devices are broadly used as display panels for apparatuses used in various applications because of their many features, such as their thin size, low power consumption and so on. Recently reflective liquid crystal display devices without requiring a backlight-unit have been developed vigorously. Attention being drawn, among them, to a light-scatter mode reflective liquid crystal display device that provides bright display without requiring a polarization plate.
As a color means for such a light-scatter mode liquid crystal panel, there is a disclosure in Japanese Patent Laid-open No. H8-184815 having a structure formed by color filters arranged on an inner surface of an upper substrate at a front side of a polymer dispersed liquid crystal layer and a reflection layer having a mirror reflection characteristic arranged at a rear side of the polymer dispersed liquid crystal layer. Further, Japanese Patent Laid-open No. H7-152029 discloses a structure having color filters arranged at a front side of a polymer dispersed liquid crystal layer and an interference filter and a scatter reflection layer arranged at a rear side of the polymer dispersed liquid crystal layer.
However, there have been disadvantages in the color technologies for the conventional reflective polymer dispersed liquid crystal display devices, as follows.
Firstly, where pixel electrodes with mirror reflection characteristics are formed on a lower substrate and a color filter on an upper substrate, ultraviolet ray irradiation is carried out through the color filter. Accordingly, it is difficult for an ultraviolet ray to reach a liquid crystal/polymerizing compound and polymerization initiator filled between the two substrates, resulting in insufficient polymerization in the polymerizing compound and polymerization initiator. Due to this, there has been a problem that the liquid crystal is so low in light scattering property that practical quality in display is not available.
More specifically, in the case of a structure wherein each pixel is divided in three by RGB three-primary color filters, if an ultraviolet ray is irradiated through these color filters to cause phase-separation in the liquid crystal and polymer thereby forming a polymer dispersed liquid crystal layer, the polymer dispersed liquid crystal layer is greatly different in light scattering property between the respective colors due to a large difference in ultraviolet ray transmissivity for the respective color filters. For example, the pixel with good light scattering properties has brightness not varied by viewing directions. However, the pixel with poor light scattering properties has brightness that is extremely light in a positive reflecting direction and dark in directions other than that. Due to this, there has been a problem that color balance deviates depending upon a viewing direction resulting in poor quality in display. Meanwhile, if the ultraviolet ray transmissivity is matched to one another for the RGB color filters, there occurs color deviation in the visible light range thus resulting in degradation in color formation and hence poor quality in display. Therefore, it has been impossible to satisfy to match the ultraviolet ray transmissivity for the color filters and to maintain the color forming level for the color filters at the same time.
In order to solve such a problem, there is a proposal for a method that a color filter and a reflective layer having a mirror reflection characteristic are adjacently formed on the lower substrate at a side contacting with a liquid crystal. In this method, no filter is formed on the upper substrate so that the polymerizing compound and polymerization initiator can be polymerized homogeneously and fully through irradiating an ultraviolet ray from above the upper substrate. In this method, however, the color filter is present at a back of the polymer dispersed liquid crystal layer. Accordingly, there is increase in ratio of the light backwardly scattered by the polymer dispersed liquid crystal layer in front of the color filter to the incoming light, due to backward scattering by the polymer dispersed liquid crystal layer. Thus there has been a basic problem that the color to be displayed is lowered in color purity.
Secondly, Japanese Patent Laid-open No. H8-184815 discloses a structure and manufacture method of a reflective liquid crystal display device as a method to solve the above-stated problem. This disclosure example irradiates an ultraviolet ray from a side of the lower substrate on which no color filter is formed in order to phase-separate between the liquid crystal and the polymer. Thereafter, a reflection layer (mirror surface) is arranged on the ultraviolet-ray-irradiated lower substrate at a surface of the lower substrate not contacted with the liquid crystal. Accordingly, the ultraviolet ray can evenly and sufficiently reach the polymerizing compound and polymerization initiator, thus polymerizing the polymerizing compound and polymerization initiator to an even and sufficient extent. This provides phase separation that is homogeneous and favorable in state between the liquid crystal and the polymer for the color filters. As a result, the polymer dispersed liquid crystal layer obtained is homogeneous and stable in light scatteration and is favorable in color reproduction and contrast for the color filters. However, in this method there occurs a problem that is serious in display quality called "parallax" (or double image) because of the placement of the reflection layer (mirror surface) at an outside of the lower substrate. Here, the parallax refers to a phenomenon of double image that a shade (virtual image) is viewed overlapedly with an actual image. Where a glass exists between the color filter with polymer dispersed liquid crystal layer and the reflection layer (mirror surface), parallax (double image) occurs due to the presence of a distance (i.e., distance between the actual image and the virtual image due to the mirror surface) proportional to the thickness of the glass.
Thirdly, the polymer-dispersed mode liquid crystal display elements as light-scatter mode display elements include, as conventionally disclosed in Japanese Patent Laid-open No. H7-152029, a liquid crystal display element of a micro-capsuled structure having low molecular nematic liquid crystal involved in high molecular micro-capsules and a liquid crystal display element of a polymer matrix structure having low molecular nematic liquid crystal involved in a porous polymer matrix. In these polymer dispersed liquid crystal display elements in a structure having liquid crystal droplets within the polymer, liquid crystal molecules are placed along polymer walls of liquid crystal droplets. The liquid crystal molecules existing near the polymer walls suffer strong affection by the polymer walls. Due to this, the molecules, if applied by a voltage, will not be oriented in the electric field direction but is directed at a certain angle with respect thereto. Due to this, where the incoming light is oblique in direction, a difference occurs between the refractive index in an axial direction of the liquid crystal molecule and that of the polymer, leaving somewhat unavoidably light scatteration. Thus, there has been a problem that no favored transparent state is obtained even if a high voltage is applied. Also, if the cell gap is increased to enhance the light scatteration, light scatteration tends to occur for obliquely incoming light during application of voltage, resulting in whity cloudiness in display color with difficulty of vivid color representation and further decrease in contrast. Also, in the droplet-type polymer dispersed liquid crystal display element there is a method to decrease the cell gap in order to decrease drive voltage. However, if the cell gap is decreased to for example 10 .mu.m or smaller, the light scatter state is worsened to an extent of not reaching a practical-use level. Accordingly, it has been difficult for the droplet-type polymer dispersed liquid crystal display element to obtain respective states of sufficient light scatteration and transparency with good contrast. Due to this, if the conventional droplet-type polymer dispersed liquid crystal display element with color filters and a reflection layer are combined to structure a reflective liquid crystal display device as in the Japanese Patent Laid-open No. H8-184815, there is insufficiency in either light scatteration or transparency thereby resulting in occurrence of variation in characteristics particularly due to a difference in filter colors. For example, where the light scattering is insufficient, the light scattering is worse in red as compared with blue thus resulting in worsened viewing-angle color balance. Meanwhile, where the transparency is insufficient, opaque in blue as compared with red thereby worsening viewing-angle black level. Thus there has been a difficulty in being mounted for practical application on a small-sized portable appliance with reduced voltage and power requirement.
Fourthly, in the case of an active matrix drive that active elements are arranged to drive display elements having pixels each divided in three by RGB three-primary color filters, if a polymer dispersed liquid crystal layer is formed by phase-separation between the liquid crystal and the polymer through irradiation an ultraviolet ray via the color filters that are different in ultraviolet ray transmissivity by the color, there results large variations in the voltage holding characteristic, bake characteristic, light resistance and so on for the polymer dispersed liquid crystal layer corresponding to the color filters. Specifically, there has been a problem that flicker or baking occurs in a particular color due to the variations in these characteristics.